Interlaced video comprises of fields, each of which may be captured at a distinct time interval. A pair of fields, for example, a top field and a bottom field may be considered to comprise a frame. The pictures forming the video comprise a plurality of ordered lines. During one of the time intervals, video content for the even-numbered lines may be captured. During the other time interval, video content for the odd-numbered lines may be captured. The even-numbered lines may be collectively known as the top field, while the odd-numbered lines may be collectively known as the bottom field. Alternately, the odd-numbered lines may be collectively known as the top field, while the even-numbered lines may be collectively known as the bottom field.
In the case of progressive video frames, all the lines of the frame may be captured during one time interval. Interlaced video may comprise fields that were converted from progressive frames. For example, a progressive frame may be converted into two interlaced fields by organizing the even numbered lines into one field and the odd numbered lines into another field.
When encoding interlaced video, i.e. video that consists of a sequence of fields; an encoder may have the ability to choose between coding individual fields as field pictures and coding pairs of adjacent fields as frame pictures. One method may be more efficient than another for any given frame or field to be encoded. When encoding frame pictures, an encoder may utilize coding methods that treat frames as progressive video. These are generally referred to as progressive coding tools. It is desirable to make the optimum choice between field and frame coding for every field or every pair of fields. In cases where there is little or no motion between adjacent fields, weaving two fields together to form a frame and encoding the resulting frame using progressive coding tools may be the preferred choice. Additionally, some interlaced video content may comprise 3:2 pulldown patterns, wherein for example 24 frames per second progressive video has been converted to 60 fields per second interlaced video. Typically the 3:2 pulldown video comprises a pattern wherein some fields from the original content are repeated two fields after their first occurrence, forming TBT (top bottom top) or BTB (bottom top bottom) patterns. When encoding video comprising a 3:2 pulldown pattern, it is desirable to detect and reverse the 3:2 pulldown pattern to reconstruct the video, for example, 24 frames per second progressive video before encoding, and to encode the resulting frames of video using progressive coding tools. It is desirable to detect transitions between normal interlaced video and 3:2 pulldown video, and vice versa, as quickly as possible. Additionally, some interlaced video content may comprise other patterns, such as 2:2 pulldown patterns, wherein progressive frames of content have been converted to interlace. In such cases it is desirable for an encoder to detect and reverse the pattern and reconstruct the original progressive frames of video, and to encode the resulting frames using progressive coding tools.
Traditional 3:2 pulldown detectors measure the similarity of fields of the same polarity, for example, top (T) and T or bottom (B) and B, two fields apart, and may utilize this similarity measure to decide whether the current video input follows a 3:2 pulldown pattern or not. In 3:2 pulldown, the first and third fields of the TBT and BTB frames are theoretically identical, although they are affected by noise. Other pairs of like-polarity fields normally have some inter-field differences, except where there is no motion, no lighting changes and no noise. This approach of testing like-polarity fields may not work for many cases of 3:2 pulldown. Even with only 3:2 pulldown and interlaced formats, comparison of only like-polarity fields with no other measurements may result in a significant delay in making reasonably accurate determination of transitions between 3:2 pulldown and interlace formats, with resulting artifacts.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.